rustup: add completions

[kamilwaz]

Add completions for cargo too (if exists)

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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[SMillerDev]

Why doesn't this generate everything?

[kamilwaz]

By default, this command generates only a script for rustup. It also supports cargo, but it needs to be called separately.

Here’s how it looks in practice:

$ rustup completions zsh | grep compdef
#compdef rustup
    compdef _rustup rustup
$ rustup completions zsh cargo | grep compdef
#compdef cargo